苦
く · ku · JLPT N3 · noun
trouble
🌿 苦 = 艹 grass + 古 old: old herbs turn bitter, and bitterness means trouble
苦 first meant a bitter herb, and that meaning grew into hardship and worry. You see it in 苦労 (くろう, hardship) and 苦しい (くるしい, painful). For the sound, think of the cry ku! when something goes wrong.
苦 — grass radical 艹 + 古 (old)
Originally referred to a bitter-tasting plant, with the grass radical 艹 indicating its botanical nature and 古 providing the phonetic element. The meaning extended from the literal bitter taste to figurative senses of hardship, suffering, and painful effort.